Roll Steer

Just like bumpsteer changes the steering angle of the front tires, roll steer changes the angle of the rearend housing, resulting in understeer or oversteer. This can happen for several different reasons. Imagine a leaf-spring car hustling hard around a left-hand turn. Since the right-side spring increases in overall length as it compresses, which lengthens the wheelbase on that side of the car, the rearend will point to the right, inducing roll oversteer. For street cars, a slight degree of roll understeer results in more predictable handling, and this can be accomplished by relocating the lower control arm in cars with a four-link or torque-arm style rear suspension. Angling the lower control arms downward shortens up the wheelbase on the right side of the car when turning left, which increases understeer. Pointing the lower control arms upward has the opposite effect. As an alternative solution, Hot Rods to Hell’s unique center-drive truck arm system uses a crossmember positioned 51 inches in front of the rear axle centerline. “Since the lower control arms are so long and anchored to the center of the car, there is no change in wheelbase as the suspension compresses,” Steve McClenon explains. “This system eliminates roll steer and binding. The central mounting also eliminates chassis flex and results in incredible forward bite.”

Roll Stiffness

A suspension’s ability to resist body roll is known as roll stiffness. In essence, it is simply the combined stiffness of the springs and sway bars. Certain applications call for stiff springs and smaller sway bars, while softer springs and larger sway bars are more prevalent in other applications. For cars that see both street and track duty, many suspension tuners prefer a spring that is barely stiff enough to prevent a car from bottoming out. This affords a more compliant ride quality, and enables the tires to more closely follow the contours of the road. “The most important factor is determining how a car will be used. For a street car that sees occasional track time, we recommend softer springs and larger sway bars,” Heidts’ Mike Hawley explains. “If you’re setting up a car that will only be driven on the autocross, you can get away with stiffer springs and smaller sway bars. If that’s still not stiff enough, you can make the sway bars even bigger. Every application is different, so we ask our customers lots of questions to make sure we know their goals for their car. We can get things pretty close right off the bat, but we will let our customers swap out springs if necessary.”

Scrub Radius

Like bumpsteer, scrub radius is something that must be minimized for optimal handling. Let’s say you’re looking at the front suspension while standing directly in front of a car. If a line was drawn through the upper and lower ball joint, the angle this line creates with the ground is the kingpin inclination. If a second line was drawn from the wheel centerline to the ground, the distance between it and the kingpin inclination line is the scrub radius. Bumps and cornering loads transmitted through the tires exert a twisting force on the steering system that’s proportional to the length of the scrub radius, so it’s imperative to keep it to a minimum. Consequently, cars with zero scrub radius can be driven easily without power steering. “Scrub radius can be kept to a minimum by keeping the ball joints as close to the wheel centerline as possible,” RideTech’s Bret Voelkel says. “You can also affect scrub radius significantly by changing wheel offset. Most of the time, you end up with a scrub radius that’s in an acceptable range by special coincidence.”